Simoun, The Sky Crawlers Recommendations
Both anime feature child pilots who fly aircraft during a war in alternate universes.
The Sky Crawlers is set in a world similar to ours with some differences. Simoun is set in a more fantasy-type world
In The Sky Crawlers, the pilots stay as children for as long as they live, while in Simoun, the pilots can choose when they want to become adults.
They both focus on drama and development, touching on philosophical and ethical issues related to the respective worlds that the characters of these two anime live in.